In general, rubber materials are more shock absorbing than metals and plastics. Of the rubber materials, silicone rubber is expected to find use in a wider variety of fields because its heat resistance, weather resistance and electrical insulating properties are excellent. However, the rubber material mainly absorbs impact energy by way of distortion due to its viscoelasticity and thus requires a thickness in the impact applied direction, which represents a certain weight. Under the current trend of promoting weight reduction in such applications as transporting vehicles, business machines, and electric appliances, there is a desire to have a material which is lightweight and sufficiently elastic and permits a number of molded parts to be manufactured.
One known material is silicone rubber foam. The silicone rubber foam is typically prepared by adding pyrolytic blowing agents. Alternatively, a silicone rubber composition is molded and cured in such a manner that hydrogen gas may evolve upon curing as a by-product whereby a foam is obtained. However, the addition of pyrolytic blowing agents has the problem that decomposition gases are toxic and odorous. Where a platinum catalyst is used as the curing catalyst, undesirably the blowing agent acts to retard curing. The method of utilizing hydrogen gas evolving upon curing suffers from the problems that hydrogen gas is explosive and the uncured composition requires careful handling during storage.
Silicone rubber foam can also be formed by injection molding a silicone rubber composition in a mold whereby the rubber is expanded in the mold. This method is difficult to produce a silicone rubber foam having uniform micro-cells.